Using a DSP Controller to Control A Three-Phase Induction Motor

Transcription

1 Session 2533 Using a DSP Controller to Control A Three-Phase Induction Motor Richard E. Pfile, Maher Rizkalla Indiana University-Purdue University at Indianapolis Abstract A new course was developed jointly by EE and EET departments at IUPUI to teach power systems that are used in electric vehicles. The course content includes an overview of electric vehicles and has modules that cover batteries, power electronics, motors, and three-phase induction motor control in detail. It has weekly laboratories and a required laboratory project that extends for several weeks. This paper concentrates on the induction motor control module, but gives an overview of the entire course as well. In the portion of the course that examines motor control, students program a DSP controller in C language to control a three-phase induction motor. The new Texas Instruments TMS320C24x DSP controller with a space vector PWM peripheral is used. This device provides a single-chip solution to three-phase induction motor control. All students in the course write an open loop motor control program in C language. Students write software to setup a time base and control a motor by calculating the vector components of the space vectors that switch power transistors of a 3-phase inverter. Special low overhead programming techniques such as sin table lookup must be used to program a fixed-point processor so that it executes fast enough to continuously generate motor outputs in real-time. Students, who choose motor control as their laboratory focus area, are required to implement a closed loop fuzzy logic motor speed control algorithm. The fuzzy logic block examines the motor load, slip angle, and velocity error to determine the next output. The three-phase induction motor control techniques are presented in this paper. I. Introduction A course titled Design of Electronic Instrumentation for Electric Vehicles was developed with funding from Department of Education s FIPSE program to teach electric vehicle technology to junior and senior EE and EET students at IUPUI. Engineering and technology students took the same exams and did the save laboratory exercises, but in the course group projects the EET and EE students would assume project roles that best fit their disciplines. This is the only course in the school that is common for both the EE and EET majors. The applied nature of the course makes it suitable for both programs. Page

2 The course is interdisciplinary and integrates many different technologies that range from power electronics, computer simulation, data acquisition, DSP/microprocessor induction motor control, and fuzzy logic. Normally each of these topics is covered in detail in separate courses, but by using an only as needed philosophy, we can cover what we need from these fields that serve an important application: Electric Vehicles. 1 The course has weekly laboratories and four group projects. All students are required to complete the weekly laboratories and select one of the group projects. This paper focuses on the series of weekly laboratories in induction motor control. II. Background Variable Speed Motor Control Only in the recent years have ac motors been used in industry for variable speed applications. Because variable speed control is easily implemented with dc machines, they have dominated in variable speed applications. However, ac machines offer many advantages over dc machines. They are smaller than dc machines with the same horsepower, have a lower initial cost, are more reliable and have lower maintenance costs than dc machines. AC machines, however, have one major disadvantage; speed control of ac machines is much more complex than dc machine speed control. In the past, this has limited ac machine speed control to areas where speed regulation was not critical. The introduction of low cost and high performance DSP chips provides a useful control platform for variable speed ac-induction motors. Principles of ac induction Motor Control With proper input control, a three-phase inverter can generate sinusoidal power suitable for controlling a variable speed ac-induction motor from a dc source. The inverter consists of three half bridges (figure 1) that connect the outputs to either V dc or ground. If an upper transistor in figure 1 is enabled the output is at level V dc, if a lower transistor is enabled the output is at V Gnd. Turning on both the upper and lower transistors in the same path will produce a short to ground, damaging the inverter. The three inverter outputs are connected to the induction motor inputs. By switching the control transistors in the proper sequence a three-phase power input to an induction motor can be generated. DSP processors have fast cycle execution speeds, low interrupt latencies, and sophisticated internal timers making them ideal as a source for the control signals to an inverter. The voltages produced across the phases of a motor are best explained by example. Turning on inverter transistor a and turning off transistor a in figure 1 sets the input voltage to winding A of the motor to V dc. Likewise if transistor b is turned on and b is turned off, V dc is provided to winding B. If inverter transistor c is turned off and transistor c is turned on, winding C of the motor is connected to ground. In this configuration, the line-to-neutral voltage drops across windings A and B are 1/3 V dc with 2/3 V dc across winding C. Table 1 shows the inverter input combinations (a, b, and c), the voltages to the windings (V a, V b, and V c ), and the corresponding line-to-neutral voltage drops across the windings (V an, V bn, and V cn). In the inverter-input column of Page

3 table 1, a one turns an upper transistor on and a lower one off, and a zero does just the opposite. The first row in table 1 corresponds with the situation when all three coils of the motor are connected to ground and the voltage drops across the windings are all zero. The last row in the table has all three legs connected to V dc and again the voltage drops across the windings are all zero. The other six combinations in the table provide power to the motor. By cycling the combinations in the proper order, voltages across the motor windings approximate a sinusoidal input. In figure 2 V an is shown for the following inverter input sequence: 100, 110, 010, 011, 001, and 101. Additionally, the voltages applied to windings A, B and C are 120 out of phase with each other, providing a threephase input. Figure 1. Inverter and Motor a b c V a V b V c V an V bn V cn V dc -1/3 V dc -1/3 V dc 2/3 V dc V dc 0-1/3 V dc 2/3 V dc -1/3 V dc V dc V dc V dc -2/3 V dc 1/3 V dc 1/3 V dc V dc 0 0 2/3 V dc -1/3 V dc -1/3 V dc V dc 0 V dc 1/3 V dc -2/3 V dc 1/3 V dc V dc V dc 0 1/3 V dc 1/3 V dc -2/3 V dc V dc V dc V dc Table 1. Winding voltages produced for inverter input combinations. Since six steps are required to generate one period of each of the input sine waves, each step represents a 60 revolution of the stator flux. The speed of the motor is controlled by how fast the controller rotates the flux through the pattern at U x (figure 3) to the pattern at U x+60. To control this rate a time-base must be set up. This is Page

4 accomplished using a DSP timer that counts to a specified terminal and interrupts the processor so a routine that updates the motor position gets executed at a specified rate. 2/3 Vdc 1/3 Vdc -1/3Vdc -2/3 Vdc Figure 2. Voltages across winding A Figure 3. Switching patterns To synthesize a sine wave input, the controller can repeatedly switch back and forth from one output combination to a following one in the sequence. By properly setting the duty cycle between enabling the U x and U x+60 outputs from the inverter, a close approximation to a sine wave can be output to the motor phases. 2 The objective of sinusoidal PWM is to synthesize the motor currents as near to a sinusoid as economically possible. The power delivered to the motor can be decreased, by applying the 000 or 111 output combinations as shown in table 1. Both of these combinations deliver no power to the motor. By switching in either of these combinations, power delivered to the motor is lowered. Page

5 The TI 320C24x chip was selected for this course because it has peripheral hardware that simplifies the induction motor control problem. The processor has a register that accepts a three-bit pattern and will automatically switch power between a pattern at U x and U x+60 in proportion to two counts loaded into compare registers. Any time remaining on the control period will be an off time to set power delivered to the motor. At every update interval the programmer merely enters a voltage pattern and two counts to control the pattern output times. 3 III. Motor Control Laboratories Laboratory Exercises All students in the course are required to complete a series of three weekly laboratories leading to open loop speed control of a three-phase ac motor. In the first exercise students write a simple C program to learn how to use the compiler, linker, and debugger. In the next laboratory exercise, students write the code to setup a timer to interrupt program execution and execute a subroutine at a fixed interval. This sets up the time base required for processor based speed control systems. An interrupt rate of 10,000 interrupts/second was selected, which is the motor control update rate used in the final exercise. When using this update rate, a motor turning at 2000 RPM will get new control inputs at every 1.2 of revolution. Programming the timer requires setting up a timer control register and programming a count in the timer period register. The timer counts at a 20 MHz rate and works by counting up from zero to the terminal count loaded and then counting back to zero at which time an interrupt is generated. This following C language code loads the timer period register and sets the timer mode. /* timer interval=2*1000*50ns=100 us or 10 khz */ tipr = 1000; /* mode is continuous up/down count, use internal processor clock */ t1con = 0x6842; Code needs to be executed in response to the timer interrupt. Setting up an interrupt requires that general system interrupts be enabled and the specific timer interrupt get enabled. To demonstrate that the system was working students wrote a program that counted 10,000 interrupts and then toggled an LED. The following code enables the timer interrupt and clears the interrupt flag in the interrupt routine. /* enable the timer 1 interrupt, write a one to unmask */ evimra = 0x80; /* clear the timer interrupt flag in the interrupt routine */ evifra = 0x80; Page

6 In the next laboratory code was added to provide open loop speed control. It is necessary to generate the output for the U 0 and the U 60 axis. If α is the angle of the motor output vector shifted into the 0-60 sector, M sin(60-α) and M sin(α) generate the proper outputs for the U 0 and U 60 axis respectively. 4 A sine table is used to generate the sine values. The sine table is 100 entries long and represents sine values for angles from 0 to 60 degrees. The following code accesses a sine table. Variable SinIndex provides the proper index into the sine table. The lookup is accomplished by simply accessing an element in an array. Ta = sin_data[100 SinIndex]; Tb = sin_data[sinindex]; The outputs from the sine table lookups are eventually used to set the time counts for the U 0 and U 60 outputs. The two time counts can be loaded into the space vector PWM peripheral on the TI chip and proportional power will be delivered to the U 0 and U 60 outputs. The C language modulus operator, %, returns the remainder from an integer division. It is used to map an angle anywhere in a 360 circle to an angle shifted into the first 0-60 sector. The following code fragment uses the C language modulus operator to map any angle into the first 0-60 sector. In the fractional notation 32,767 represents a full motor revolution or 360. SixtyAngle = Angle % 32767; In an open loop control mode such as a blower drive, the effective voltage delivered to the motor is nearly linear with respect to frequency. At very low frequencies the relationship ceases to be linear. A lookup table is used to maintain a proper relationship between voltage and frequency. The following code scales the output duty up with motor speed. A fractional scaling is achieved by multiplying by an integer and then dividing by a scale value found in a lookup table. The table lookup is an access into an array. /* Lookup a scale value VoltageCurve = VoltageData[MotorSpeed]; /* Multiply by the fraction 10/VoltageCurve */ Tb = (Tb*10)/VoltageCurve; Once Ta and Tb are calculated they are entered into PWM peripheral registers and control the enable times for the axis at angles U x and U x+60. A command must also be entered into the space vector PWM peripheral to set the vector space for the proper 60 sector in the 360 rotation sequence. Page

7 Laboratory Equipment A Texas Instruments (TI) TMS320C24x EVM board was the platform used to execute the software. A PC was used for software development and downloading programs to the TI EVM board. Spectrum Digital manufactures a board that facilitates interfacing the TI EVM with an inverter. This interface board provides logic to protect against sending signals that will simultaneously turn on both transistors in one half-bridge of the inverter and damaging it. The unit also provides jumper selectable output drive levels and outputs with sufficient current to drive most inverters. The inverter used was also from spectrum digital and can be powered from ac line voltage or from a dc source. Several different fractional horsepower motors were used for the laboratories. A block diagram of the system is shown in figure 4. IBM PC TI EVM PWM Spectrum Digital Interface PWM Inverter Current, Velocity Phase Currents 3 Phase Control AC Motor Figure 4. Laboratory station setup The group project for the motor control module was to implement a closed loop IV. Fuzzy Logic Group Project The group project for the motor control module was to implement a closed-loop speed control system using fuzzy logic. Input variables to the fuzzy control software were motor current, slip angle, and speed. The fuzzy control software had outputs that modified the output duty cycle to increase or decrease power to the motor and to change the motor speed by modifying the rate of change of the vector angle. Five output fuzzy Page

8 levels were used: large increase, small increase, no change, small decrease, and large decrease. The student group was divided into groups that performed analysis, design, and system implementation. Engineering students worked in the analysis and design, while technology students coded and implemented the designs. V. Programming Techniques To update the motor control at a rate of 10,000 hertz requires that the software calculates and output a new motor control command in no more than 100 microseconds. The TI processor is a fixed-point processor and responding to control in the time period allotted precludes using floating point calculations. All angles used for the motor control are based on using integer numbers as a fraction of the full-scale value of a 16-bit number. When scaling numbers, variables are multiplied and then divided by two integers that form the proper scaling ratio rather than multiplying by floating point numbers. This allows all calculations to be completed using integer arithmetic. Again there is not enough time to calculate a sine value using a typical C library function call. To get the sine of an angle a sine table is made again with the full-scale 16- bit value equal to a 1. To get a value out of the table the code accesses the table with an array index that indicates the data point into the sine table. VI. Conclusion In the course students receive sufficient exposure to a variety of fields to obtain an adequate background to search the literature on their own and pursue engineering design in the area. Although the motor control module lasted only four weeks, students had a good grasp of the concepts as demonstrated by their ability to complete a group project using fuzzy logic control. The group projects gave students an opportunity to engage in a group learning process and become well versed in a particular specialty. 1 Maher E. Rizkalla, Richard E. Pfile, Ahmed El-Antably, and Charles F. Yokomoto, Development of a Senior Elective for EE and EET Majors in the Design of Electronic Instrumentation for Electric Vehicles, ASEE National Conference, Seattle, Washington, June 28-July 2, J. Holtz, Chapter 4 Power Electronics and Variable Frequency Drives, IEEE Press, Piscataway, NJ, 1996, pp TMS320C24x DSP Controllers Peripheral Library and Specific Devices-Reference Set Volume 2, Texas Instruments Incorporated, 1997, pp Zhenyu Yu and David Figoli, AC Induction Motor Control with TMS320C240 Using Constant V/Hz Principle and Space Vector PWM Technique, DSP Digital Control Systems Applications Texas Instruments, Houston, TX. pp Richard E. Pfile is a professor of Electrical Engineering Technology at IUPUI. He received his B.S. from the University of Louisville and his M.Eng. from the University of Michigan. He has won the Outstanding Teaching Award and has received Teaching Excellence awards from the School of Engineering and Technology at IUPUI. He teaches courses in microprocessor systems, computer networks and digital signal processing. He has 15 years of teaching experience and eight years of industrial experience, including three years as a systems engineer. Page

9 Maher E. Rizkalla is a professor of Electrical Engineering at IUPUI. He received his Ph.D. from Case Western Reserve University in Electrical Engineering in His current research interests include VLSI design as applied to DSP, electromagnetics, solid state electronics, and applied engineering education. He received two FIPSE grants a NSF ILI grant, and many other industrial grants. He is a registered professional engineer in the State of Indiana. Page